I sent some ideas to Mari when they were on on...Saturday I believe? Namely in reference to organizing the shops better and migrating away from "big-medium-small" style so players can more easily find what they're looking for instead of browsing literally every shop to find, say, Beacons. My suggestions in that regard was to either A) Eliminate player-owned shops period by having assigned shop-sections in the District that focus more on item-type versus player (think actual Mall or Supermarket). This method would be easier for players to browse ANYONE selling something they're looking for in an easy to navigate section. This method would also be more easily managed in terms of clean-up from inactive or rarely-restocked members; no more giant shops of half-empty, or incomplete construction projects mucking up the District. A staff member would simply have to pick a day of the week/month to go through x-section and its chests to clear them out. This method, additionally, enables players who do sell items to offer a more direct and competitive market. There wouldn't be a price-cap or minimum but the range and availability based on shopper-need would encourage more exposure for players who need large numbers of some items, such as harder to come by blocks like Quartz or red netherbrick and stuff.
Option B) maintains the more complicated and build-diversity options of player-built and managed shops, which means harder to discourage inactivity or never-stocked shops. The alteration here, though, is providing an actual document and map with markers so players can actually FIND the shops with the items they want by linking the book information with the marker on the map. This option requires a dedicated player or staff to pretty much man the shops alone and make certain new shops get added, old ones get removed, and product availability is updated.
Option C) again includes the map and book info, but also incorporates a bit of option A. option C) would allow players to still make their own shop, however, they would be restricted to District sections that are MUCH more clearly outlined in the area than the current arrangement. These districts would be like Nether supplies, End supplies, Potion crafting, Enchantments (books and gear), etc. Locked in paths and "Zoning" would prevent overlap and players going outside their restricted zone, and if players want to change the sort of zoned item they're selling, they would need to dismantle their current shop completely. No more enormous every-item shops dispersed across the entire district, and no selling allowed until you have actual stock to sell.
For prolonging playtime, I disagree with Fire. Eliminating keepinv is as much a voting-reward as it is an incentive TO stay on the server. I can't tell you the number of times I've eventually stopped doing anything on a server at all simply because I ended up losing all my stuff for an asenine reason. I'm a builder, not a PVPer, or explorer. I don't mind the exploring part on occassion, but I hate pvping, and I hate the risk when I've working for hours or days or weeks or more on something, only to lose everything and not be able to even have the chance at recovering it.
In regards to the elytra you suggested @Fir3Z3r0 I actually agree with this one. The End can be as dangerous as the nether. but if you're just joining the server, have kitted up, and are on the hunt for Elytra...it's a pain in the ass when your competition has the ability to firework and fly for thousands of blocks without consequence, loot everything, and move on all while the non-elytra player is still crossing the void from portal to first island.
Setting a price-min wouldn't work well, tbh. Like has been said, it's too easy to undercut. There's some players, like myself, who trade for labor or with other materials when wanting something. I conserve my diamonds for the build project I'm working on and don't like spending dozens of blocks or more of them just to get certain items. I'm frugal like that. If I can trade for it, I will.
Speaking of Trade....
That in itself could be one way to help maintain a higher integrity and value to the shops. Eliminate common-item shops entirely. Preserve the SHopping district for higher-tier, mid to late game items like potions, enchanted things, obsidian and beacons, and so forth. You would still have high-competition from players who use the shops more often trying to sell their higher-tiered stuff, but much less clutter from common items like stones, woods, etc. For those sorts of things, set up an actual trade system where players looking to buy certain stuff not in the district can request it and what they have to trade/are willing to do for it, and members can do like a command of /trades or whatever to review who's wanting what. No more chat-spamming "Who has this/Who sells this?" and no more District clutter.